Libvirt incremental backups use named checkpoints to track which guest-disk blocks changed after a backup point. To create one, verify that the host supports the workflow, then run virsh backup-begin with backup XML that refers to an existing checkpoint; optionally provide checkpoint XML to establish the next point. Libvirt’s documented support is limited to QEMU with qcow2 disks at the active layer, so confirm your actual configuration and host capabilities before relying on it.
What a libvirt incremental backup does
A checkpoint marks a point from which libvirt tracks changes to guest-disk blocks. A later incremental backup can use that checkpoint to identify changed portions rather than treating the operation as a new full backup. The QEMU driver implements this tracking with block dirty bitmaps. A checkpoint is tracking metadata—not a copy of the previous disk contents, a VM snapshot, or a record of guest memory. Your recovery plan must therefore retain the backup data needed to reconstruct a usable restore, not just checkpoint metadata. See libvirt’s explanation of incremental backup handling in QEMU.
Check support on the target host first
Libvirt’s checkpoint documentation states: “Right now, incremental backups are only supported for the QEMU hypervisor when using qcow2 disks at the active layer.” This is a specific support statement, not a guarantee for every libvirt or QEMU release. The official pages do not give a complete version matrix; verify the installed versions and the domain’s actual disk format and active layer. The domain capabilities XML reports whether backup and checkpoint features are supported by the host for a domain.
- Confirm the domain uses QEMU and that each disk intended for incremental backup meets the documented qcow2 active-layer requirement.
- Inspect the target host’s reported backup and checkpoint capabilities rather than assuming support from the presence of virsh commands.
- Check the installed libvirt and QEMU documentation and behavior for your release, since the cited documentation does not establish a universal minimum version.
Choose push or pull mode
The backup XML selects the data flow. Push is the default: the hypervisor writes backup data to the configured destination. Pull mode exposes an NBD server so a third-party client can read the backup data; it requires temporary scratch storage, typically local, until the backup completes. The official XML guide describes these modes and their configuration, but does not establish general performance, consistency, or security advantages for either. Choose according to the client, destination, access controls, and recovery workflow you operate.
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| Mode | Data flow | Destination handling |
|---|---|---|
| Push (default) | The hypervisor writes the backup data. | Configure the destination in backup XML. |
| Pull | An NBD endpoint is made available for a third-party client to read. | Configure the NBD server and provide scratch storage until the operation completes. |
For XML elements and examples, use the libvirt Backup XML format guide. Adapt examples to the domain’s disks, destination paths, permissions, and restore requirements; they are syntax illustrations, not a ready-made configuration for every host.
Create an incremental backup
The command pattern documented by virsh is virsh backup-begin DOMAIN BACKUP.xml CHECKPOINT.xml. Replace DOMAIN with the domain name or identifier and use XML files that reflect your environment.
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- Identify the base checkpoint. The backup XML must name an existing checkpoint in its
<incremental>element for an incremental operation. Confirm that the checkpoint and the backup data it represents are part of your retained backup chain. - Configure the backup XML. Set the top-level mode to
pushorpullas appropriate; push is the default. Specify which disks to include or exclude and configure targets. Per-disk settings can override the overall mode or checkpoint, so inspect each disk entry rather than assuming all disks behave alike. - Optionally prepare checkpoint XML. Pass a checkpoint XML file as the third argument when you want to create a new checkpoint with the backup. This establishes the point for a subsequent incremental backup.
- Start the job. Run
virsh backup-begin DOMAIN BACKUP.xml CHECKPOINT.xml, omitting the final argument if you are not creating a new checkpoint during this operation. Consult the installed virsh manual for argument details applicable to your release. - Monitor or stop the asynchronous job. The backup runs asynchronously. The virsh manual documents checking push-job progress with
virsh domjobinfo DOMAINor waiting for an event; usevirsh domjobabort DOMAINto end a job when necessary. Confirm completion before treating the destination as a finished backup.
Without a backup XML file, virsh backup-begin defaults to a full push backup. That default is not an incremental operation from a checkpoint. Command behavior is documented in the virsh manual.
Preserve checkpoint lineage and plan retention
Each incremental operation depends on the checkpoint it names and on the backup data required to restore from the resulting chain. Keep checkpoint metadata and corresponding backup files aligned in your retention and deletion procedures. The virsh manual warns that deleting checkpoint metadata can remove the ability to back up incrementally from that point. Deleting a checkpoint is therefore not equivalent to deleting an unneeded label if later backups depend on it.
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Transient domains need special care: the virsh documentation says checkpoint metadata is silently lost when a transient domain quits. If the domain lifecycle does not preserve that metadata, do not assume the checkpoint will remain available for a later incremental backup.
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